1. Field of the Invention
The present invention relates to pre-aligning a substrate in a lithographic apparatus, a lithographic apparatus containing an alignment system containing a pre-alignment device configured to perform a pre-alignment method, a device manufacturing method, and a device manufactured according to the device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a manufacturing process using a lithographic projection apparatus, a pattern in a patterning device is imaged onto a substrate positioned on a substrate support. The substrate is at least partially covered by a layer of energy-sensitive material (also termed resist). Prior to, and after this imaging step, the substrate may undergo various procedures, such as a post-exposure bake (PEB), development, a hard bake, and measurement and/or inspection of the imaged features. These procedures are used as a basis to pattern an individual layer of a device, e.g., an integrated circuit (IC). Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate. These devices are then separated from each other by a technique such as dicing or sawing, after which the individual devices can be mounted on a carrier, connected to pins, etc.
Alignment is the process of positioning the image of a specific point on the patterning device to a specific point on the substrate that is to be exposed. Typically, one or more alignment marks, such as a small pattern, are provided on each of the substrate and the patterning device. As indicated above, a device such as an IC may consist of many layers that are built up by successive exposures with intermediate processing steps. Before each exposure, alignment is performed to minimize any positional error between the new exposure and the previous ones, such error being also termed overlay error. A substrate generally may be provided with alignment marks at both sides of the substrate.
An alignment process involves a pre-alignment procedure in which the edge and the center of a (generally circular) substrate are determined, and an alignment procedure in which the substrate is accurately aligned using the one or more alignment marks provided on the substrate. After a substrate has been pre-aligned in a pre-alignment device, the substrate is transferred to a substrate support by a substrate handler, which usually is a robot having an arm for transferring the substrate from the pre-alignment device to the substrate support.
In a pre-alignment procedure, different devices may be used. In a known substrate pre-alignment procedure, a pre-alignment device is used having a pre-alignment support provided with mechanical pins abutting predetermined edge portions of the substrate. Use of such a pre-alignment device presupposes a known diameter of the substrate. If the diameter of the substrate may vary, an offset in the determination of the orientation of the center of the substrate may be introduced, both in terms of an offset in an X direction, an offset in a Y direction, and an offset in a φ (angular) direction (where the substrate essentially extends in an X-Y plane, and φ denotes an angle of rotation of the center of the substrate). In fact, the offset defines a difference between the substrate's geometrical coordinate system and a coordinate system based on the alignment marks provided on the substrate. The offset may be determined when the diameter of the substrate is known. Alternatively, a pre-alignment device may be used having a substrate edge detector relative to which the substrate is rotated on a pre-alignment support of the pre-alignment device. By rotating the substrate and simultaneously measuring the location of the edge, a center X, Y and φ offset may be determined. As a further alternative, a pre-alignment device may be used having a number of spaced substrate edge detectors located along an expected edge portion of a substrate. No rotation of the substrate is necessary to determine a center of substrate, although, if the diameter of the substrate may vary, an X, Y, and φ offset in the determination of the orientation of the center of the substrate may be introduced. These offsets may be determined when the diameter of the substrate is known.
Once the offset referred to above has been determined, an orientation of the particular substrate can be chosen such that the substrate may be placed correctly on a substrate support to perform an alignment procedure.
Following a pre-alignment procedure, in an alignment procedure of a substrate on a substrate support, the image of an alignment mark on the patterning device is positioned accurately to an image of an alignment mark on the substrate.
It is observed that in some technologies, such as micro system technology (MST) and micro electromechanical systems (MEMS), devices are fabricated from both sides of a substrate. Exposures performed on one side of the substrate must be accurately aligned with features previously exposed on the other side of the substrate.
Throughout this specification, reference to an alignment mark being on a particular side of the substrate of course includes the alignment mark being etched into that side of the substrate, and includes the alignment mark having subsequent material deposited on top of it such that it is embedded and is no longer necessarily exposed at the surface.
U.S. Patent Application Publication No. 2002/0109825 A1, which hereby is incorporated by reference in its entirety, discloses a lithographic apparatus which is provided with an optical system, such as a system employing one or more laser beams, built into a substrate table for producing an image of an alignment mark that is provided on the backside of the substrate, i.e., the side of the substrate which is facing the substrate table. The image is located at the plane of the front side of the substrate, and can be viewed by an alignment system from the front side of the substrate. Simultaneous alignment between marks on the back and front of the substrate and a patterning device can be performed using a pre-existing alignment system.
In a production facility where substrates are processed to yield devices, lithographic equipment of different suppliers may be used. The different suppliers generally employ different pre-alignment procedures to determine the edge and the center of the substrate prior to an alignment procedure based on the recognition of one or more alignment marks on the substrate. Therefore, unexpected or at least basically unknown position deviations of the alignment marks may be encountered when processing substrates on different equipment. Moreover, when using one or more alignment marks (sometimes also referred to as alignment targets) on the backside of a substrate, there is a restriction in the positioning of such alignment marks. This restriction stems from the limited physical size of a view window of the optical system used for determining the position of the alignment marks. A pre-alignment method therefore should be designed to place a substrate on a substrate support such that the alignment marks on the backside of a substrate may be viewed in the view window of the optical system, despite the use of lithographic equipment from different suppliers used to produce specific devices.